1. Field of the Invention
This invention relates to cellulosic polymer compositions that can be formed into membranes, films, hollow fibers and other similar structures. An aspect of this invention relates to the use of these polymers as permselective membranes. Another aspect of this invention relates to the synthesis of perfluoroacyl modified cellulose acetates.
2. Description of the Prior Art
Cellulose, as is known in the art is a polymer made up of repeating saccharide (anhydroglucose) units linked at the 1 and 4 positions by betaglycocidic bonds. Numerous cellulosic polymers are known including cellulose acetate, carboxymethyl cellulose, hydroxyethyl cellulose, ethyl cellulose and so on. Perhaps no conventional polymer has found more applications in industrial membrane separations than cellulose acetate. In various forms it is, for example, used in sea water desalination, ultrafiltration, microfiltration, kidney dialysis and gas separations.
In the early 1960's Drs. Loeb and Sourirajan discovered a special preparative technique for casting cellulose acetate membranes whose structure consists of an ultrathin permeable skin formed on the top of a gelled porous support matrix. Such a membrane provides more permeant fluxes, i.e. the ability to maintain a rapid transmission of liquid or gas therethrough, than those obtained from conventional dense cellulose acetate membranes.
However, it is difficult to cast other polymeric materials in modified structures by the Loeb and Sourirajan technique. Even for cellulose acetate, the technique requires elaborate experimental procedure, and for gas separations the cellulose acetate membranes need to be carefully dried before use via freeze drying techniques or solvent exchanges. Furthermore, cellulose acetate membrane does not always give satisfactory results in field applications using the Loeb and Sourirajan technique. Decreases in selectivity, i.e. the ability to retain a solute and pass through a solvent, and flux are noticed as the membranes age. The cause of such failures varies with the application. In reverse osmosis it is normally attributed to compaction of the porous gelled structure of the membrane at high operating pressures, which may range from 100 to 2,000 pounds per square inch. In gas separations it is attributable to defects caused during drying of the normally wet membrane, and also to defects caused by exposure to water vapors and other polar solvents at high pressure.
An alternative to the Loeb-Sourirajan technique, well known in the art, involves the lamination of an ultrathin polymer layer to a porous support medium wherein the layers are made of different material. In function this composite membrane is similar to the Loeb-Sourirajan type membranes, but because it is comprised of two distinct laminas which can be tailored independently for their specific purposes, it offers much greater flexibility in construction and matching end use requirements.
Initial research herein involved the use of this modified technique utilizing cellulose acetate, however such attempts were not successful. Cellulose acetate, as other researchers have noticed, does not prove to be a good ultrathin film former. Thicker films were not quite as useful due to their lower flux.
Membranes with improved characteristics, made from perfluoroacylated ethyl cellulose polymers have been reported. See U.S. Pat. No. 4,008,047 (Petersen) issued Feb. 15, 1977.
Petersen demonstrated that when perfluoroacyl groups are added to ethyl cellulose, through substitution of the residual hydroxyl groups, the resultant polymers can be cast into ultrathin membranes with superior performance abilities. The perfluoroacyl ethyl cellulose polymer membranes were found to have improved blood compatability properties, and other desireable characteristics for polymers used in membrane applications.
In addition to Petersen others have reported the synthesis of cellulose polymers. British Pat. No. 1,120,373 (Park et al), published July 17, 1968 and (Ghatge et al) Journal of Applied Polymer Science (1984). Park et al briefly discusses synthesis of perfluoro esters of a variety of cellulose derivatives including cellulose acetate by reaction with perfluoroacyl halides. However, Park's work did not have any membrane objectives. Park's work entailed surface modifications of clothing materials. They found that the modified polymers had lower surface energy and provided better resistance to staining and soiling when applied to clothing articles. The synthesis disclosed in Park et al did not result in a high degree of fluorination of the cellulose derivate, that content being only about one percent (1%) by weight. Recently Ghatge et al described a modified cellulose acetate polymer synthesized by reacting cellulose acetate (39.9% acetyl) polymer with pheynl isocyanate, for formation of membranes used in reverse osmosis applications. Except for Park et al the use of perfluoroacyl halides for modifying cellulose acetate is not reported.